Ahead of the G-20 meeting of world leaders, the largely peaceful protests against unchecked capitalism, war, destruction of the environment and other ills are a sign of the populist anger in Europe.

Reporting from London — Fed up with feeling powerless, Michael McKenny woke at dawn today, kissed his 2-year-old son goodbye and boarded a bus for the five-hour ride to London. It was his chance, he said, to do something constructive with his anger over the state of the world, rather than just “sitting at home whining about it.”

For several hours, McKenny joined thousands of demonstrators who thronged London’s financial district in a catchall protest against unchecked capitalism, war, the destruction of the environment and other societal ills. Closely watched by police, the protesters offered a raucous public counterpoint to what are expected to be the cozy, closed-door meetings Thursday of leaders from the Group of 20, a collection of the world’s biggest and up-and-coming economies.

“Times are changing,” said McKenny, 25. “Capitalism the way it’s been practiced is ending. . . . It needs to crash. It needs to reset.”

Today’s demonstrations in the British capital were the latest manifestation of the populist anger surging across Europe as the global recession deepens. Growing social unrest has already toppled governments in Iceland and the Czech Republic, and could do the same in other countries.

Most of the London protests were peaceful, but there were sporadic outbreaks of violence and vandalism.

A group of demonstrators smashed the windows of the Royal Bank of Scotland, which has come to symbolize, in Britain, the damaging excesses of the financial sector. Officers in riot gear were deployed in some areas, and television footage showed them swinging their batons at protesters who emerged with bloodied faces. By early evening, police said they had made two dozen arrests.

But much of the time, an almost carnival-like atmosphere reigned. Blessed with sunny spring weather, activists belonging to a loose coalition calling itself the G-20 Meltdown began marching this morning into the City, London’s financial district, in four columns. They converged on the Bank of England building bearing their versions of the Bible’s Four Horsemen of the Apocalypse — in this case, global problems such as war and climate change.

“Revolution!” some protesters shouted, while others waved placards both wrathful and whimsical: “Eat the Bankers,” “0% Interest in Others” and “Resistance Is Fertile.”

Police said late today that a man died at the protest site near the Bank of England after collapsing in a side street about 7:30 p.m. The man had stopped breathing by the time officers were able to reach him and administer CPR. He was taken to a hospital and pronounced dead.

Hundreds of miles from his home in Glasgow, 20-year-old Santino De Rosa wove his way into the thick of the crowds, past speakers blaring rock music and through air redolent of cannabis.

“To see people come out in numbers, saying we want change, we need change — that puts across a strong message,” said De Rosa, a university student. “I want a better deal for people. I want a better deal for the planet. These problems haven’t been resolved with corporations running amok.”

An estimated 4,000 to 5,000 protesters descended on the Bank of England, an imposing, 2-century-old neoclassical building. The police, out in force as part of a massive security operation costing more than $10 million, moved quickly to contain the demonstrators shortly after noon, refusing to let anyone in or out of the area for nearly three hours. The confinement led to some frayed tempers.

A few streets away, scores of environmental activists brought traffic to a halt with a makeshift tent city to protest carbon-emissions trading. And a few hundred antiwar activists gathered during the evening in Trafalgar Square, London’s heart.

Shop owners in the protest areas had boarded up their stores after police warnings in recent days that fringe groups could try to hijack the demonstrations and wreak havoc. And to avoid becoming easy targets, many employees in the City showed up for work wearing jeans and other casual clothing.

More protests are expected Thursday during the G-20 meeting, but the summit venue, on London’s east side, has been tightly secured for the event.